Component sizing with a function

So Ive tried to search but maybe I’m not using the correct verbiage. What I’m trying to do is size a component using the ‘operators’ function of - (minus or subtract) where the amount to be subtracted is static.

I have an opening of 24" and need the component to clear with .5" on each side, thus the component will be 1" smaller.
The LenX attribute should look something like 24"-1" but this does nothing.

You might wanna upload what you have got?

Check this wiki: